Triggerable vacuum arc device suitable for alternating current operations

ABSTRACT

Triggerable vacuum arc devices are actuated by a single laterally disposed trigger assembly adjacent the arcing gap electrically isolated from the arc electrode when not actuated.

United States Patent [191 Lafferty June 11, 1974 TRIGGERABLE VACUUM ARC DEVICE SUITABLE FOR ALTERNATING CURRENT OPERATIONS [75] Inventor: James M. Lafferty, Schenectady,

[73] Assignee: General Electric Company,

Schenectady, N.Y.

[22] Filed: Aug. 21, 1972 [2]] Appl. N0.: 282,606

Related US. Application Data [63] Continuation of Ser. No. 72,684, Sept. 16, 1970,

abandoned.

[52] US. Cl. 315/330, 313/198 [51] Int. Cl. H01j 13/48 [58] Field of Search 313/198, 197; 315/330 [56] References Cited UNITED STATES PATENTS 3,328,632 6/1967 Robinson 313/198 X 3,448,337 6/1969 Kawiecki 313/198 X 3,450,922 6/1969 Gallagher 313/198 X Primary Examiner-Nathan Kaufman Attorney, Agent, or FirmPatrick D. Ward; Joseph T. Cohen; Jerome C. Squillaro [5 7] ABSTRACT Triggerable vacuum are devices are actuated by a single laterally disposed trigger assembly adjacent the arcing gap electrically isolated from the arc electrode when not actuated.

1 Claim, 4 Drawing Figures PATENTEDJUN 1 1 m4 $816798 sum 2 or 2 IN l E N TOR.

JAMES M. LAFFERTY,

HIS ATTORNEY TRIGGERABLE VACUUM ARC DEVICE SUITABLE FOR ALTERNATING CURRENT OPERATIONS This is a continuation, of application Ser. No. 72,684,

filed Sept. 16, 1970, now abandoned.

The present invention relates to triggerable vacuum arc devices adapted to change, in very short time intervals, from a first non-conducting state to a second conducting state, carrying extremely high currents at very high voltages. More particularly, the invention relates to such devices which are polarity-independent and may readily be used in alternating current installations, as well as in direct current installations.

Triggerable vacuum arc devices have been known for sometime. In my US. Pat. No. 3,087,092, entitled Gas Generating Switching Tube, issued Apr. 23, 1963, l disclose and claim such a device as is adapted to render a normally open, non-conducting arc-gap conducting upon the application of a suitable voltage pulse to a trigger assembly which initiates a trigger are which produces ionized specie to render the main gap conducting. In subsequent improvements of devices of this type, the permissible operable currents and voltages which may be handled thereby have been increased and a high degree of reliability and extended lifetime have been achieved.

Most triggerable gas devices, however, suffer from failure due to eventual and premature destruction of the trigger assembly. This is due in part to the rather fragile nature of the trigger assembly as compared with the massive rugged construction of the primary arcelectrodes and due also in part to the general practice of locating the trigger assembly within one or both of the arc electrodes in such a way that, when a trigger arc is initiated and subsequently, a primary arc is established, the trigger is exposed to the corrosive and erosive effects of the main breakdown current. The location of the trigger adjacent one of the arc electrodes very often within a recess in the cathode electrode is due to the fact that one function of the trigger assembly is believed to be the establishment of the cathode spot on the cathode electrode to help establish an arc. Thus, it has been found that if the trigger assembly is located adjacent the arc electrode, the trigger voltage neces sary to break down the main gap may be as high as ten times the voltage required when the trigger assembly is adjacent the cathode electrode. In direct current (DC) systems, it is no problem to locate the trigger which is adjacent (generally recessed within) the cathode electrode. Although the trigger assembly is subject to are erosion thereat, if the appropriate steps are taken by recessing and by shielding the trigger assembly within the cathode electrode, these suitable protective measures make such location acceptable.

For alternating current (AC) operation, the problem becomes more difficult. For complete insurance of breaking down the primary gap, a trigger assembly may be located in each arc electrode so that on any given half cycle, a trigger assembly is associated with the cathode electrode so that breaking down of the gap and the formation of a cathode spot is greatly facilitated. Although from the breakdown point of view, such a location appears ideal, it is no way an easy solution, for destructive anode spots may form on the anode electrode and upon the trigger associated therewith and may destroy the device by destroying one trigger assembly thereof in the very first operation. On the other hand, it is imperative for the protection of most equipment, that the current be rendered harmless thereto (generally by short circuiting the line temporarily) on the same half cycle in which an over voltage occurs due to lightening or transient line fluctuations, for example.

Generally, the solution to the problem, involves the use of two oppositely poled, parallel-connected Triggerable Vacuum Arc (TVA) devices such as a Triggered Vacuum Gap (TVG) Device or Triggerable Vacuum Switch (TVS) Devices, and the associated circuitry therefor. The disadvantages of such arrangements as to increased cost, increased requirements for space and complexity of such arrangements are obvious.

Furthermore, in such instances in which attempts have been made to provide a trigger in such a way as to solve many such problems, as for example, in my US. Pat. No. 3,276,169, issued Sept. 6, 1966, wherein a plurality of ring shaped primary arc electrodes surround a trigger assembly located centrally thereof, it has been found that the solution is decidedly not optimum. Thus, for example, in my aforementioned patent, not only is the trigger assembly subject to the possibility of damage by an anode spot, but it has been found that ring shaped electrodes are highly susceptible to highly destructive anode spot formation due to the hanging up of the arc upon such electrodes in the absence of electromagnetic means to rotate the arc.

Accordingly, it is an object of the present invention to provide triggerable vacuum devices suitable for operation on both DC and AC systems.

Another object of the present invention is to provide a single unitary triggerable vacuum arc device which is adapted to operate on AC systems to become conducting upon the occurrence of a suitable pulse thereto irrespective of the voltage polarity on the arc electrodes thereof at any predetermined time.

Still another object of the present invention is to provide a triggerable arc device adapted for either DC or AC operation and having only a single trigger assembly.

A further object of the invention is to provide a triggerable vacuum arc device, and associated triggering circuitry therefor, which are adapted to short circuit a line supply voltage from protected electrical apparatus upon the occurrence of an operating voltage overload.

Briefly stated in accord with one embodiment of the present invention, I provide a triggerable vacuum arc device including an evacuable envelope containing a pair of arc electrodes which may be spaced from one another to define an arcing gap and a single trigger assembly laterally disposed adjacent the arcing gap and adapted upon being pulsed to inject charge particles into the arcing gap to cause the breakdown thereof. The trigger assembly is pulsed in accordance with an over-voltage condition and is maintained electrically isolated from both primary arc-electrodes both during the quiescent state and during arcing, after the actuation of the trigger circuit has ceased, but is electrically connected to the instantaneous cathode electrode through a low resistance during actuation of the trigger circuit.

The novel features of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following depending end of a typical trigger assembly utilized in the device of FIG. 1,

FIG. 3 is a schematic circuit diagram of one suitable trigger activator system for the device of FIG. 1, and

FIG. 4 is an alternative circuit to that illustrated in FIG. 3.

In FIG. 1 of the drawing, a triggerable vacuum switch represented generally as includes an evacuable envelope 1 1 adapted to be evacuated to a pressure of 10 torr or less, and comprising a pair of insulating cylindrical members 12, preferably of a suitable ceramic and a pair of metallic end wall members 13 and 14 having a substantially cup shaped configuration and secured to the outwardly depending ends of ceramic cylinders 12 by metal-to-ceramic seals 15. A first, fixed, primary arc electrode 16 is supported within envelope 11 on electrode-support member 17, which is mechanically and electrically affixed to cup shaped end member 13. A second primary arc electrode 18 is disposed within envelope 11 adjacent primary arc electrode 18 and supported upon a primary arc electrode support member 20 which extends into envelope 11 and is supported thereby with support member 20 passing through an aperture 21 in end member 14. The support is provided and the hermetic seal of the envelope is maintained intact by closing the aperture 14 by bellows member 22 which is affixed between the flat inwardly depending portion of cup shaped end member 14 and the inwardly depending end of second primary arc electrode support member 20 to hold are electrode member 18 in a relatively fixed position, depending from are electrode 16 to define arcing gap 19, but permitting of longitudinal motion so as to close the gap between the primary arc electrodes when desirable.

A trigger assembly 24 is hermetically sealed through the intersection of cylinders 12 and terminates at the inwardly-depending end and is supported thereat by a lateral cylindrical shield member 25. Shield member 25 is supported by flange 26 which is attached to seal collar 27 which is affixed in ceramic-to-metal seal between cylinder members 12 and is an integral portion of envelope ll. Shielding to prevent the diffusion of evaporated or sputtered metallic vapors from arcing gap 19 throughout the device and coating the interior of insulating ceramic cylinders 12 is completed by the interposition of secondary end shield members 29 and 30 which extend longitudinally inwardly into the volume of lateral shield 25 leaving only a small annular opening at either end for passage of vapors such that any vapor escaping from the interaction space defined by the shields is essentially directed upon the flat inwardly depending portions of cup end members 13 and 14.

Trigger assembly 24, illustrated in greater detail as to its important aspects in FIG. 2, comprises a first metallic throat inlet washer 32 and a metallic cup shaped member 33 which is connected to a trigger anode contact 34 which passes outwardly and is connected to the trigger pulse circuit. The active element of the trigger assembly is an insulating washer 36, preferably of ceramic, as for example, alumina or beryllia ceramic. Ceramic washer 36 is prepared by first metallizing the inner and end, but not the outer surface thereof, by techniques well-known to the metal-to-ceramic seal arts, and the coating thereupon of a high vapor pressure metal capable of withstanding bakeout temperatures of, for example, 600800C without decomposition but yet having a sufficiently high vapor pressure, i.e., that equal to or in excess of the vapor pressure of copper but not as high as vapor pressure of magnesium. Preferably, copper, beryllium, nickel, or tin or a suitable alloy thereof, may be utilized as the metallic coating for washer 36. A trigger gap 37 is scored completely around the inner surface of the metallic coating on washer 36 to define a pair of gap-adjacent metallic coated portions 38 and 39. Film portion 38 is in contact with metallic throat washer 32, which is in contact with shield member 25. Film portion 39 is in contact with metallic cup member 33, which is in contact with trigger anode lead 34. Connection to the trigger cathode may be made to any member connecting the shield as for example, to member 27, or to the casing 28 of the trigger assembly.

Although the trigger cathode is connected to shield member 25, the trigger cathode and trigger anode, insofar as the structure of the device illustrated in FIG. 1 is concerned, are electrically isolated from one another and from both of the primary arc electrodes 16 and 18. This is in contradistinction to the generalized situation in devices having arc-electrodes which are axially located along the longitudinal axis of a cylindrical device housing, wherein the trigger assembly is usually physically located within one or more arc electrode members and the trigger cathode is electrically and mechanically affixed to the arc electrode member to facilitate the formation of a cathode spot.

In operation, the device of FIG. 1, functions essentially as a complete electrical protective agency for an electrical apparatus. Thus, for example, let it be assumed that large capacitor banks are utilized to correct the power factor of an inductive transmission line. The capacitors must be protected against lightening strikes to the line and from transient over-voltages in line voltage, irrespective of the cause. For such protection, the device of FIG. 1 may be placed in parallel with the capacitors across the line and an appropriate circuit may be utilized placing the trigger circuit either across the line through appropriate circuitry or it may be connected with appropriate transformers which sense a sample of the line voltages. Upon the occurrence of a line over-voltage, the trigger is actuated and a sufficient voltage is applied between film members 38 and 39 surrounding trigger gap 37 to cause the breakdown thereof. The heating by trigger current bridging gap 37 causes the release of ionized metallic specie from the high vapor pressure metal constituting film members 38 and 39 and the injection of the charged particles into the primary gap. A cathode spot is established and the primary gap becomes conducting. The trigger pulse ceases and the main pulse is carried momentarily by the primary arc support members and by the primary arc members. Normally, the current will be interrupted upon the occurrence of a current zero of the half cylce on which the arc is struck, or upon one or two cycles thereafter. If the arc is continuously restruck after approximately three cycles, then passage of electric current through the primary arc electrode circuit is generally utilized to actuate a closer mechanism causing arc electrode support member 20 to be urged forward against the spring tension of bellows 22 bringing arc electrodes 16 and 18 into contact and, thereafter, current is carried without arcing, so long as the overvoltage condition exists. This closing is usually accomplished by an integrating actuation circuit which senses an extended fault-current. Upon cessation .of the overvoltage condition and the fault-current, a second appropriate sensing means may then be caused to withdraw support member 20, withdrawing arc electrode 18 from are electrode 16, thus returning the device to the quiescent state.

The entire operation described hereinbefore ordinarily, in alternating current circuits, takes place within a few voltage half cycles. Thus, the initial breakdown of the triggered gap of the triggerable gap causes the near instantaneous closing of the switch and upon the approach of-an appropriate current zero in the alternating current, the fault may be cleared and the device returned to the quiescent state. Alternatively, should an extended fault occur, the device is adapted to react to protect the arcing surfaces.

In FIG. 3 of the drawing, a circuit suitable for actuation of the device of FIG. 1 is illustrated. The TVA device is represented at and are electrode 16 and 18 are appropriately identified. Trigger assembly 24 is connected between a pair of spark or equivalent breakdown gap devices 40 and 41 through a pair of current limiting resistors 42 and 43. A pair of high resistances may be connected across gaps 40 and 41 to insure equal voltages across both, independent of gap leakage currents. When an over-voltage occurs across line terminals 44 and 45, a voltage V/2 exists across each of breakdown gap devices 40 and 41. If the voltage V is of sufficient magnitude to actuate the gap devices, the gaps become conducting and a voltage V/2 is developed across each of resistors 42 and 43. The voltage between the trigger assembly and the primary arcelectrodes is then +V/2 one electrode and V/2 for the other. When the trigger arc is established, positive ions (charged metallic particles from the film of the trigger electrode) are accelerated from the trigger to the more negative of the two primary arc electrodes, and the impingement thereupon causes the formation of cathode spots which facilitate the formation of a primary arc. Upon the establishment of the primary are between the arc-electrodes, even prior to the possible, but not necessary, mechanical closure of the primary arcelectrodes, the trigger current ceases due to the higher impedance of the trigger circuit as compared with the primary arc circuit.

The great advantage of the illustrated apparatus and its mode of operation as is described with respect to FIG. 3 is that the single trigger apparatus, although maintained in electrical isolation from both primary arc electrodes when no trigger current is flowing, i.e., when the device is in the quiescent state and also when the primary arc is burning, is nevertheless electrically connected through a suitable current limiting resistance to that electrode which is to be the primary arc cathode upon excitation of the primary arc. This greatly facilitates the establishment of cathode spots thereupon. In this mode of operation, and because of the particular structure and components utilized, the advantages of the prior art location of the trigger within the arc electrode namely the ready formation of cathode spots is attained. Simultaneously, the disadvantageous aspect thereof, namely the susceptability of the trigger to the erosive effects of the primary are, are avoided. Thus, this device is able to function effectively to establish cathode spots to facilitate main arc breakdown, while at the same time rendering the trigger safe from unnecessary deterioration. All of this is done with the simultaneous achievement of substantial economy in cost and space by utilizing only a singly trigger assembly to excite and activate either electrode to operation as a primary arc cathode, depending upon the polarity of the main voltage upon pulsing of the device to operation.

In FIG. 4, an alternative trigger excitation system is illustrated utilizing a current transformer 46 to isolate the trigger pulse from line terminals 44 and 45. In spite of this, the connection of the trigger to the primary arc electrode during arcing is facilitated by connection of the trigger to arc electrode 18 through spark gap 40, although no connection is provided to are electrode 16 through spark gap 41, and none is needed.

Although the invention has been described herein with respect to particular configurations, it will be readily apparent to those skilled in the art that the illustration of both the apparatus and of the circuits is largely schematic and that many other configurations may be substituted. The important characteristics of the invention are the utilization of a substantially butt type pair of primary arc electrodes surrounded by an electrically floating shield which has, as a portion thereof, a trigger assembly which is laterally disposed with respect to the arcing gap so as to inject the charge of ionized specie into the gap without being exposed thereto. Similarly, the characteristics of the pulsing circuit are that the trigger assembly is isolated from the primary arc electrodes during quiescent condition of the switch and the primary arc current but that the trigger assembly is connected to the cathode electrode through a current limiting resistance and a broken down gap device during the flow of trigger circuit, in order to cause the formation of a cathode spot to facilitate the establishment of the primary arc.

While the invention has been set forth herein with respect to certain embodiments thereof, many modifications and changes will readily occur to those skilled in the art.

Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Triggerable vacuum arc apparatus comprising: an evacuated envelope; first and second primary arc electrodes each having an arcing surface located within said envelope, said arcing surface being spaced apart from each other to define therebetween a primary arcing gap within said envelope; a first shield member mounted within said envelope and encompassing at least said arcing surfaces and said primary arcing gap therebetween, said first shield member having an aperture therethrough which is located proximate said primary arcing gap, second and third shield members encompassing portions of the first and second electrode support members, respectively, portions of said second and third shield members being encompassed by said first shield member; a trigger assembly comprising first and second annular trigger electrodes located within said envelope proximate said aperture in said first shield member, said first shield member located bepassing through said aperture in said first shield member, the spacing between said annular electrode defining a trigger gap; and terminal means electrically connected to said first annular electrode and extending in sealed relationship outside said envelope. 

1. Triggerable vacuum arc apparatus comprising: an evacuated envelope; first and second primary arc electrodes each having an arcing surface located within said envelope, said arcing surface being spaced apart from each other to define therebetween a primary arcing gap within said envelope; a first shield member mounted within said envelope and encompassing at least said arcing surfaces and said primary arcing gap therebetween, said first shield member having an aperture therethrough which is located proximate said primary arcing gap, second and third shield members encompassing portions of the first and second electrode support members, respectively, portions of said second and third shield members being encompassed by said first shield member; a trigger assembly comprising first and second annular trigger electrodes located within said envelope proximate said aperture in said first shield member, said first shield member located between said primary arcing gap and said trigger electrodes, said second annular electrode being in electrical contact with said first shield member, said first and second annular trigger electrodes being spaced apart from each other along a central axis passing through central openings in said annular electrodes as well as passing through said aperture in said first shield member, the spacing between said annular electrode defining a trigger gap; and terminal means electrically connected to said first annular electrode and extending in sealed relationship outside said envelope. 